Adding manual thresholds to plots that can have individual labels, colour and linestyles

[borjasor]

It would be a nice and useful feature to be able to add thresholds (horizontal and vertical) to figures when no EPICS channels exist for this purpose. Currently the advice is to use 'hline' and 'vline' however there is no support to specify the colour, linestyle and label to them, for instance in the way of; 'hline-linestyle', 'hline-labels', 'hline-color'